Absorption Refrigeration Machine

ABSTRACT

An absorption refrigeration machine with a working medium, containing a nonvolatile sorption medium and a volatile refrigerant, and an absorber in which a vapor phase containing refrigerant and a liquid phase containing sorption medium are separated from one another by a semipermeable membrane which is permeable to the refrigerant and impermeable to the sorption medium.

The invention is related to an absorption refrigeration machine having areduced mechanical energy requirement.

Classical refrigeration machines are based on a circuit in which arefrigerant is vaporized and cooling is achieved as a result of the heatof vaporization taken up by the refrigerant. The vaporized refrigerantis then brought to a higher pressure by means of a compressor andcondensed at a higher temperature than in the vaporization, resulting inthe heat of vaporization being liberated again. The liquefiedrefrigerant is subsequently depressurized again to the pressure of thevaporizer.

The classical refrigeration machines have the disadvantage that theyconsume a large amount of mechanical energy for compression of thegaseous refrigerant.

In comparison, absorption refrigeration machines have a reducedmechanical energy requirement. Absorption refrigeration machines have,in addition to the refrigerant, the vaporizer and the condenser of aclassical refrigeration machine, a sorption medium, an absorber and adesorber. In the absorber, the vaporized refrigerant is absorbed at thepressure of the vaporization in the sorption medium and is subsequentlydesorbed again from the sorption medium in the desorber at the higherpressure of the condensation by introduction of heat. The compression ofthe liquid working medium composed of refrigerant and sorption mediumrequires less mechanical energy than the compression of the refrigerantvapor in a classical refrigeration machine; the consumption ofmechanical energy is replaced by the thermal energy used for desorptionof the refrigerant.

U.S. Pat. No. 1,882,258 discloses an absorption refrigeration machine inwhich a liquid phase composed of the absorption medium water and therefrigerant ammonia is separated in the absorber by a porous permeablewall from a gas phase containing gaseous ammonia and an inert gas. Themachine described in this document is intended to be operated withoutsupply of mechanical energy. In the machine described in this document,absorber, desorber, condenser and vaporizer are all operated at the samepressure, which results in a low efficiency. In addition, not onlyammonia but also water vapor can get from the liquid phase through theporous wall into the vaporizer and lead to malfunctions there, e.g. as aresult of ice formation.

The same problem occurs in the process proposed in DE 633 146, in whichthe absorber is provided with a diaphragm through which the absorptionmedium water can likewise get in vapor form into the absorber, and alsothe process proposed in WO 2004/104496 which likewise uses water assorption medium.

DE 195 11 709 discloses an absorption refrigeration machine having aworking medium containing LiBr or LiBr/ZnBr₂ as nonvolatile sorptionmedium and water or methanol as volatile refrigerant. Condensation andvaporization of the refrigerant do not occur in this machine. Instead,cooling is effected by pervaporation of refrigerant from arefrigerant-rich working medium, coming from the absorber, through ahydrophobic membrane into a refrigerant-deficient working medium, comingfrom the desorber. In this machine, the membrane is not located in theabsorber.

U.S. Pat. No. 4,152,9001 and U.S. Pat. No. 5,873,260 disclose absorptionrefrigeration machines in which the desorber is provided with a membranethrough which desorption of refrigerant from the working medium occurs.In the process of U.S. Pat. No. 4,152,9001, sorption medium passesthrough the membrane and gets into the vaporizer. In U.S. Pat. No.5,873,260, too, it is stated that the sorption medium water passes asvapor through the membrane.

Accordingly, there continues to be a need to further reduce themechanical energy requirement compared to prior art absorptionrefrigeration machines, without malfunctions occurring as a result ofabsorption medium getting into the vaporizer of the refrigerationmachine.

The invention provides an absorption refrigeration machine having aworking medium, containing a nonvolatile sorption medium and a volatilerefrigerant, and an absorber, in which a vapor phase containingrefrigerant and a liquid phase containing sorption medium are separatedfrom one another by a membrane, wherein the membrane is a semipermeablemembrane which is permeable to the refrigerant and impermeable to thesorption medium.

Arrangement of a semipermeable membrane, which separates the gaseousrefrigerant from the liquid working medium, in the absorber incombination with a nonvolatile sorption medium allows absorption evenagainst a pressure difference between the gaseous refrigerant and theliquid working medium, without sorption medium being able to get intothe vapor space of the absorber and into the vaporizer. As a result, therefrigerant can be absorbed even at a pressure of the liquid workingmedium which is significantly higher than the pressure of thevaporization without any compression of the gaseous refrigerant beingnecessary and without malfunctions occurring as a result of sorptionmedium getting into the vaporizer. The mechanical energy requirement forcompression of the liquid working medium can be reduced compared to theabsorption refrigeration machines known from the prior art by anincrease in the pressure of the liquid working medium in the absorber.

The absorption refrigeration machine of the invention comprises aworking medium, containing a nonvolatile sorption medium and a volatilerefrigerant, and a semipermeable membrane which is permeable to therefrigerant and impermeable to the sorption medium. In principle, it ispossible to use all working media, having a nonvolatile sorption mediumand a volatile refrigerant, which are known from the prior art forabsorption refrigeration machines, in combination with a suitablesemipermeable membrane.

The working medium preferably contains water, ammonia or carbon dioxideas volatile refrigerant. Water is particularly preferably used asrefrigerant.

The working medium additionally contains a nonvolatile sorption medium.In this context, the term nonvolatile sorption medium refers to asorption medium whose vapor pressure is a factor of more than 10 000lower than the vapor pressure of the refrigerant. Preference is given tosorption media which at 20° C. have a vapor pressure of less than 10⁻³mbar, particularly preferably less than 10⁻⁶ mbar. Preference is givento using polymeric or salt-like sorption media as nonvolatile sorptionmedia.

A working medium which is known from the prior art and is suitable forthe absorption refrigeration machine of the invention is an aqueoussolution of lithium bromide with water as refrigerant and lithiumbromide as nonvolatile sorption medium.

A sorption medium comprising one or more ionic liquids is preferablyused in the absorption refrigeration machine of the invention. In thiscontext, the term ionic liquid refers to a salt or a mixture of saltscomposed of anions and cations, where the salt or the mixture of saltshas a melting point of less than 100° C. Compared to conventionalsorption media such as lithium bromide, ionic liquids have the advantagethat a larger proportion of the refrigerant can be driven off in thedesorber without solidification of the sorption medium. Using an ionicliquid as sorption medium, the absorption refrigeration machine cantherefore be operated without malfunctions at a higher capacity of theworking medium for the refrigerant, which allows more compactconstruction of the absorption refrigeration machine.

The ionic liquid preferably consists of one or more salts of organiccations with organic or inorganic anions. Mixtures of a plurality ofsalts having different organic cations and the same anion areparticularly preferred.

Suitable organic cations are, in particular, cations of the generalformulae (I) to (V):

R¹R²R³R⁴N⁺  (I)

R¹R²N⁺═CR³R⁴   (II)

R¹R²R³R⁴P⁺  (III)

R¹R²P⁺═CR³R⁴   (IV)

R¹R²R³S⁺  (V)

where

-   -   R¹, R², R³, R⁴ are identical or different and are each hydrogen,        a linear or branched aliphatic or olefinic hydrocarbon radical        having from 1 to 30 carbon atoms, a cycloaliphatic or        cycloolefinic hydrocarbon radical having from 5 to 40 carbon        atoms, an aromatic hydrocarbon radical having from 6 to 40        carbon atoms, an alkylaryl radical having from 7 to 40 carbon        atoms, a linear or branched aliphatic or olefinic hydrocarbon        radical which has from 2 to 30 carbon atoms and is interrupted        by one or more —O—, —NH—, —NR′—, —O—C(O)—, —(O)C—O—, —NH—C(O)—,        —(O)C—NH—, —(CH₃)N—C(O)—, —(O)C—N(CH₃)—, —S(O₂)—O—, —O—S (O₂)—,        —S (O₂) —NH—, —NH—S (O₂)—, —S (O₂)—N (CH₃)— or —N (CH₃)—S (O₂)—        groups, a linear or branched aliphatic or olefinic hydrocarbon        radical which has from 1 to 30 carbon atoms and is terminally        functionalized by OH, OR′, NH₂, N(H)R′ or N(R′)₂ or a polyether        radical of the formula —(R⁵—O)_(n)—R⁶ having a block or random        structure,    -   R′ is an aliphatic or olefinic hydrocarbon radical having from 1        to 30 carbon atoms,    -   R⁵ is a linear or branched hydrocarbon radical containing from 2        to 4 carbon atoms,    -   n is from 1 to 200, preferably from 2 to 60,    -   R⁶ is hydrogen, a linear or branched aliphatic or olefinic        hydrocarbon radical having from 1 to 30 carbon atoms, a        cycloaliphatic or cycloolefinic hydrocarbon radical having from        5 to 40 carbon atoms, an aromatic hydrocarbon radical having        from 6 to 40 carbon atoms, an alkylaryl radical having from 7 to        40 carbon atoms or a —C(O)—R⁷ radical,    -   R⁷ is a linear or branched aliphatic or olefinic hydrocarbon        radical having from 1 to 30 carbon atoms, a cycloaliphatic or        cycloolefinic hydrocarbon radical having from 5 to 40 carbon        atoms, an aromatic hydrocarbon radical having from 6 to 40        carbon atoms or an alkylaryl radical having from 7 to 40 carbon        atoms,    -   where at least one and preferably all of the radicals R¹, R², R³        and R⁴ is different from hydrogen.

Cations of the formulae (I) to (V) in which the radicals R¹ and R³together form a 4- to 10-membered, preferably 5- to 6-membered, ring arelikewise suitable.

Heteroaromatic cations having in the ring at least one quaternarynitrogen atom bearing a radical R¹ as defined above, preferablyderivatives of pyrrole, pyrazole, imidazole, oxazole, isoxazole,thiazole, isothiazole, pyridine, pyrimidine, pyrazine, indole,quinoline, isoquinoline, cinnoline, quinoxaline or phthalazinesubstituted on the nitrogen atom, are likewise suitable.

Suitable inorganic anions are, in particular, tetrafluoroborate,hexafluorophosphate, nitrate, sulfate, hydrogensulfate, phosphate,hydrogenphosphate, dihydrogenphosphate, hydroxide, carbonate,hydrogencarbonate and the halides, preferably chloride.

Suitable organic anions are, in particular, R^(a)OSO₃ ⁻, R^(a)SO₃ ⁻,R^(a)OPO₃ ²⁻, (R^(a)O)₂PO₂ ⁻, R^(a)PO₃ ²⁻, R^(a)COO⁻, R^(a)O⁻,(R^(a)CO)₂N⁻, (R^(a)SO₂)₂N⁻ and NCN⁻ where R^(a) is a linear or branchedaliphatic hydrocarbon radical having from 1 to 30 carbon atoms, acycloaliphatic hydrocarbon radical having from 5 to 40 carbon atoms, anaromatic hydrocarbon radical having from 6 to 40 carbon atoms, analkylaryl radical having from 7 to 40 carbon atoms or a linear orbranched perfluoroalkyl radical having from 1 to 30 carbon atoms.

In a preferred embodiment, the ionic liquid comprises one or more1,3-dialkylimidazolium salts, where the alkyl groups are particularlypreferably selected independently from among methyl, ethyl, n-propyl,n-butyl and n-hexyl. Particularly preferred ionic liquids are salts ofone or more of the cations 1,3-dimethylimidazolium,1-ethyl-3-methylimidazolium, 1-(n-butyl)-3-methylimidazolium,1-(n-butyl)-3-ethylimidazolium, 1-(n-hexyl)-3-methylimidazolium,1-(n-hexyl)-3-ethylimidazolium and 1-(n-hexyl)-3-butylimidazolium withone of the anions chloride, acetate, methylsulfate, ethylsulfate,dimethylphosphate or methylsulfonate.

In a further preferred embodiment, the ionic liquid comprises one ormore quaternary ammonium salts having a monovalent anion and cations ofthe general formula (I) in which

-   -   R¹ is an alkyl radical having from 1 to 20 carbon atoms,    -   R² is an alkyl radical having from 1 to 4 carbon atoms,    -   R³ is a (CH₂CHRO)_(n)—H radical where n is from 1 to 200 and R═H        or CH₃ and    -   R⁴ is an alkyl radical having from 1 to 4 carbon atoms or a        (CH₂CHRO)_(n)—H radical where n is from 1 to 200 and R═H or CH₃.

Particular preference is given to chloride, acetate, methylsulfate,ethylsulfate, dimethylphosphate or methylsulfonate as anion.

Processes for preparing the ionic liquids are known to those skilled inthe art from the prior art.

Preference is given to using ionic liquids which are stable at atemperature of 150° C. as sorption media. When water is used asrefrigerant, preference is given to using an ionic liquid which isstable to hydrolysis.

Ionic liquids stable to hydrolysis display less than 5% degradation byhydrolysis in a mixture with 50% by weight of water on storage at 80° C.for 8000 h.

Ionic liquids forming a nonideal mixture with the refrigerant withlowering of the vapor pressure of the refrigerant are preferably used assorption media.

The working medium can contain further additives in addition to thesorption medium and the refrigerant. The working medium preferablyfurther contains one or more corrosion inhibitors. Here, it is possibleto use all nonvolatile corrosion inhibitors which are known from theprior art to be suitable for the materials used in the absorptionrefrigeration machine.

In a preferred embodiment of the absorption refrigeration machine of theinvention, the semipermeable membrane in the absorber is a solutiondiffusion membrane. A solution diffusion membrane has virtually nopores. With a solution diffusion membrane, the selective permeability ofthe membrane for the refrigerant is based on the refrigerant dissolvingin the material of the membrane and diffusing through the membrane,while the sorption medium is insoluble in the material of the membrane.The suitability of a dissolution diffusion membrane for the absorptionrefrigeration machine of the invention can therefore be determined by aperson skilled in the art by simple experiments on the solubility ofrefrigerant and sorption medium in the material of the membrane.

In the case of the preferred embodiment using water as refrigerant and asalt-like sorption medium, it is possible to use as the solutiondiffusion membrane any pore-free membrane which is known to thoseskilled in the art as suitable for desalination of aqueous saltsolutions from the technical fields of dialysis, reverse osmosis andpervaporation.

The material used for the solution diffusion membrane is preferably ahydrophilic or hydrophilically functionalized polymer containingpolyvinyl alcohol, polyimide, polybenzimidazole, polybenzimidazolone,polyamide hydrazide, cellulose ester, cellulose acetate, cellulosediacetate, cellulose triacetate, cellulose butyrate, cellulose nitrate,polyurea, polyfuran, polyethylene glycol, poly(octylmethyl-siloxane),polysiloxane, polyalkylsiloxane, polydialkylsiloxane,polyester-polyether block copolymer, polysulfone, sulfonatedpolysulfone, polyamide, in particular aromatic polyamide, polyether,polyether ether ketone, polyester, polyether-urea composite,polyamide-urea composite, polyether sulfone, polycarbonate, polymethylmethacrylate, polyacrylic acid or polyacrylonitrile. It is likewisepossible to use mixtures or copolymers of two or more of these polymers.Particular preference is given to solution diffusion membranes composedof cellulose acetate, crosslinked polyethylene glycol, crosslinkedpolydimethylsiloxane or a polyester-polyether block copolymer.

In a preferred embodiment of the absorption refrigeration machine of theinvention, the semipermeable membrane in the absorber is a microporousmembrane. For the purposes of the invention, microporous membranes aremembranes which have pores extending through the membrane having aminimum diameter in the range from 0.3 nm to 100 μm. The membranepreferably has pores in the range from 0.3 nm to 0.1 μm.

Preference is given to using a microporous membrane which is not wettedby the working medium composed of sorption medium and refrigerant in theabsorber. For the present purposes, the term wetting means a contactangle between working medium and microporous membrane of less than 90°,which leads to intrusion of working medium into pores of the membrane asa result of capillary forces. The contact angle between working mediumand microporous membrane is preferably more than 120 degrees,particularly preferably more than 140 degrees. By use of a nonwettingmicroporous membrane, a flow of the liquid working medium through thepores of the membrane to the vapor side of the membrane can be preventedeven for a pressure on the side of the liquid working medium which ishigher than that on the vapor side. A person skilled in the art cantherefore determine the suitability of a microporous membrane for theabsorption refrigeration machine of the invention by determining thecontact angle between the working medium and the membrane.

In the preferred embodiment using water as refrigerant, preference isgiven to using a hydrophobic microporous membrane as semipermeablemembrane. Suitable hydrophobic microporous membranes are known to thoseskilled in the art as waterproof membranes which are permeable to watervapor in the technical field of functional clothing.

Preference is given to using hydrophobic microporous membranes composedof polyethylene, polypropylene, polytetrafluoroethylene, polyvinylidenefluoride or fluoroalkyl-modified polymers. It is likewise possible touse mixtures or copolymers of two or more of these polymers. Inorganichydrophobic microporous membranes or composite membranes comprising aninorganic hydrophobic microporous material, for example membranes whosepores are formed by silicalite or hydrophobicized silica, are likewisesuitable.

The semipermeable membrane is preferably arranged on a porous supportlayer. Arrangement on a porous support layer enables a mechanicallystable membrane unit to be achieved even when a thin semipermeablemembrane is used. This allows more rapid mass transfer through themembrane and thus a smaller and more compact design of the absorber. Thesupport layer is preferably arranged on the side of the semipermeablemembrane which is in contact with the vapor phase. Such an arrangementof the support layer leads to a reduced mass transfer resistance than anarrangement of the support layer on the side of the membrane facing theliquid working medium.

The porous support layer can comprise either inorganic or organicmaterials. The membrane is preferably arranged on a porous support layercomposed of a hydrophobic polymer, in particular a polyolefin, apolyester or polyvinylidene fluoride. The support layer can additionallycontain reinforcement, e.g. by layers of fabric.

In a preferred embodiment, the semipermeable membrane is arranged in theabsorber in the form of hollow fibers. The configuration of the membranein the form of hollow fibers allows a particularly compact constructionof the absorber and operation of the absorber at a higher pressuredifference between the vapor phase and the liquid phase in the absorber.

The absorption refrigeration machine of the invention can additionallycontain a membrane in the desorber, through which desorption ofrefrigerant from the working medium occurs. In the desorber, it is inprinciple possible to use all membranes which are suitable assemipermeable membrane for the absorber of the absorption refrigerationmachine of the invention. The use of a membrane in the desorber alsomakes it possible to use working media which tend to foam duringdesorption.

The absorption refrigeration machine of the invention can also beconfigured in the form of a multistage refrigeration machine, asdescribed in F. Ziegler, R. Kahn, F. Summerer, G. Alefeld “Multi-Effectabsorption chillers”, Rev. Int. Froid 16 (1993) 301-311.

Owing to the compact construction and the low mechanical energyrequirement, the absorption refrigeration machine of the invention isparticularly suitable for mobile use, in particular in motor vehiclesand ships. Furthermore, the absorption refrigeration machine of theinvention is also particularly suitable for the air-conditioning ofbuildings since it can be operated with particularly low noise andallows for efficient air-conditioning using solar energy.

1-12. (canceled)
 13. An absorption refrigeration machine, comprising: a)a working medium containing a nonvolatile sorption medium and a volatilerefrigerant; and b) an absorber comprising a semipermeable membranewhich is permeable to said refrigerant and impermeable to said sorptionmedium and which is capable of separating a vapor phase containing saidrefrigerant from a liquid phase containing said sorption medium.
 14. Theabsorption refrigeration machine of claim 13, wherein said refrigerantis water.
 15. The absorption refrigeration machine of claim 13, whereinsaid refrigerant is ammonia or carbon dioxide.
 16. The absorptionrefrigeration machine of claim 13, wherein said sorption mediumcomprises one or more ionic liquids.
 17. The absorption refrigerationmachine of claim 16, wherein said ionic liquids consist of salts oforganic cations with organic or inorganic anions.
 18. The absorptionrefrigeration machine of claim 17, wherein that the ionic liquidscomprise one or more 1,3-dialkylimidazolium salts.
 19. The absorptionrefrigeration machine of claim 16, wherein the ionic liquids compriseone or more quaternary ammonium salts of the general formulaR¹R²R³R⁴N⁺A⁻, where R¹ is an alkyl radical having from 1 to 20 carbonatoms, R² is an alkyl radical having from 1 to 4 carbon atoms, R³ is a(CH₂CHRO)_(n)—H radical where n is from 1 to 200 and R═H or CH₃, R⁴ isan alkyl radical having from 1 to 4 carbon atoms or a (CH₂CHRO)_(n)—Hradical where n is from 1 to 200 and R═H or CH₃ and A⁻ is a monovalentanion.
 20. The absorption refrigeration machine of claim 19, whereinsaid refrigerant is water.
 21. The absorption refrigeration machine ofclaim 19, wherein said refrigerant is ammonia or carbon dioxide.
 22. Theabsorption refrigeration machine of claim 13, wherein the semipermeablemembrane is a solution diffusion membrane.
 23. The absorptionrefrigeration machine of claim 13, wherein the semipermeable membrane isa microporous membrane which is not wetted by the working medium. 24.The absorption refrigeration machine of claim 23, wherein therefrigerant is water and the semipermeable membrane is a hydrophobicmicroporous membrane.
 25. The absorption refrigeration machine of claim24, wherein said sorption medium comprises one or more ionic liquids.26. The absorption refrigeration machine of claim 25, wherein the ionicliquids consist of salts of organic cations with organic or inorganicanions.
 27. The absorption refrigeration machine of claim 13, whereinthe semipermeable membrane is arranged on a porous support layer. 28.The absorption refrigeration machine of claim 27, wherein the supportlayer is arranged on the side of the semipermeable membrane which is incontact with the vapor phase.
 29. The absorption refrigeration machineof claim 13, wherein the semipermeable membrane is arranged in theabsorber in the form of hollow fibers.
 30. The absorption refrigerationmachine of claim 29, wherein said sorption medium comprises one or moreionic liquids.
 31. The absorption refrigeration machine of claim 30,wherein the ionic liquids consist of salts of organic cations withorganic or inorganic anions.
 32. The absorption refrigeration machine ofclaim 30, wherein the refrigerant is water.